Coalescents (also called film-forming assistants) are known per se. They are added to aqueous coating materials and bring about filming of dispersed polymer particles to form a homogeneous coating film. Adding them is required when the film-forming temperature of the binder is above the temperature of the application.
Known film-forming assistants include the following: ethylene glycol ethyl ether, ethylene glycol propyl ether, ethylene glycol butyl ether, ethylene glycol hexyl ether, diethylene glycol methyl ether, diethylene glycol ethyl ether, diethylene glycol butyl ether, diethylene glycol hexyl ether, propylene glycol n-butyl ether, dipropylene glycol n-butyl ether, dipropylene glycol methyl ether, tripropylene glycol methyl ether, propylene glycol phenyl ether, propylene glycol tert-butyl ether, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, and 2,2,4-trimethyl-1,3-pentanediol diisobutyrate (compare “Farbe+Lack”, 101 (July 1995), pp. 606-609).
In recent times, for environmental reasons, water-based coatings have become very topical. In latex coatings, based in particular on small particles of synthetic polymers such as polyacrylates, coalescents have traditionally been used in substantial quantities. These coalescents (also called filming assistants) are added to the coatings in order to improve film formation. The function derives from the plasticizing effect exerted by the coalescent on the latex particles, enabling them to coalesce and form a continuous film. This film has optimum properties after the evaporation of the water. Important in the formation of a film is the temperature known as the film-forming temperature, at which (or below which) the polymer particles coalesce to form a film. The customary coalescents lower the film-forming temperature of the polymer.
Conventional coalescents are certain esters and ethers; known technical standards are the hydroxyl ester “Texanol” from Eastman (also often called TMB; a 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate), and also “EGBE” from Union Carbide (ethylene glycol monobutyl ether).
DE 24 43 645 was published back in 1976. Explicitly disclosed therein is benzoic acid 1,2-propanediol monoester, albeit specifically in connection with amino resins. The claims are directed to “modified amino resins” which comprise compounds of the aryl-COO—X—OH type as modifiers.
U.S. Pat. No. 3,399,158 discloses diesters of C2-6 dicarboxylic acids.
U.S. Pat. No. 4,489,188 was granted in 1984. Coalescents for coatings that are disclosed therein are adducts of EO and PO with aromatic carboxylic acids such as benzoic acid.
U.S. Pat. No. 4,894,406 from 1988 discloses 2-alkyl-1,3-hexanediol alkyl esters, i.e., special hydroxyl esters, as coalescents.
U.S. Pat. No. 5,756,569 from 1996 discloses 3-alkoxypropionates as coalescents.
EP 069,839 B1 claims the use of hydroxyl ester mixtures as flow control assistants for aqueous polymer dispersions. The compounds are those obtainable by reacting propylene oxide with aliphatic C7-12 monocarboxylic acids.
So-called reactive coalescents were mentioned at an early stage, as for example in U.S. Pat. No. 4,141,864. Dicyclopentenyloxyethyl methacrylate was used therein, but was said to have odor-related drawbacks.
According to EP 599,478 A1, acetoacetates can be used as a reactive coalescent.
EP 501,614 A2 from 1991 cites esters of ketocarboxylic acids as coalescents.
The claims of European patent EP 1,169,397 B1 are directed to film-forming compositions where polymers or prepolymers are emulsified in aqueous phase and where the composition comprises a coalescent which is an ester and is characterized by the formula R—COOX. R and X in this formula are hydrocarbyl groups or substituted hydrocarbyl groups. At least one of the two radicals must comprise at least two C═C double bonds.
US 2005/0182168 A1 discloses a combination of glycol esters of vegetable oil-based fatty acids and antioxidants. Propylene glycol monoesters are a specific topic therein.
WO 00/56823 discloses film-forming compositions having an aqueous phase and a dispersed phase, the latter comprising a polymer and a coalescent. The latter is an ester subject to the structural proviso that the acid component and/or the alcohol component must comprise at least two C═C double bonds. The technical purpose of this multiplicity of double bonds is that the coalescent itself is reactively crosslinked; it is referred to as a reactive coalescent.
Claim 1 of U.S. Pat. No. 6,762,230 B2 is directed to coating compositions comprising a latex polymer and a coalescent of the formula R1—(CO—Xr—O)n—R2. In this formula, R1 is an organic group, X is a divalent organic group, the index r can be 0 or 1, the index n can be in the range from 1 to 10, and R2 is hydrogen or an organic group. Where r adopts a value of 0 and n a value of 1, and where the radicals R1 and R2 are alkyl radicals, the stated formula circumscribes fatty acid esters. It is pointed out, however, that fatty acid esters are not specifically disclosed in U.S. Pat. No. 6,762,230 B2. Instead, the coalescents of U.S. Pat. No. 6,762,230 B2 that are specifically discussed are not fatty acid esters.
Fatty acid alkyl esters are referred to in the two following texts: in EP 026,982 A1 from 1980, aqueous coating compositions with a polymer and a coalescent are claimed. The latter is to be selected from a trio of “kinds” of esters; one of these kinds is characterized by the formula R1—COOR2. In this formula, R1 is an alkyl or aryl group and R2 is an alkyl group (bar methyl). In the paragraph bridging pages 11 and 12 it is explicitly stated that in the abovementioned ester formula it is preferably the case that R1═C3-7, and R2═C4-8 (both radicals aliphatic). At page 13 lines 10-33, suitable esters are listed by name. At page 13 line 35 et seqq. it is expressly stated that methyl and esters are generally to be avoided on account of their instability (tendency toward hydrolysis) in paint systems. Here, then, the skilled person is explicitly advised against using, as coalescents, esters whose alcohol component is methanol or ethanol, and is implicitly advised against using, as coalescents, esters whose alcohol component has 3 or 4 C atoms (since, in the preferred embodiment, as mentioned above, R2 embraces a range of 4-8 C atoms). In particular there is no specific disclosure of any fatty acid esters whose alcohol component has 1 to 3 C atoms.
DE 24,49,471 A1 from 1974 claims esters of straight-chain C2-6 fatty acids, the alcohol component of the esters being aliphatic C3-4 alcohols, as solvents for coating materials. There is no reference there to coalescents.